Polyethylene is known for presenting excellent dielectric properties, and also low cost price. That is why it is nowadays in widespread use for making insulating layers of power and/or telecommunications cables.
In order to provide improved thermomechanical properties, polyethylene is generally used in a cross-linked form. It is known that establishing a lattice of chemical bonds extending in all three dimensions serves to increase the high temperature behavior of this particular type of insulating material.
Cross-linked polyethylene is usually fabricated by silane cross-linking. That now-conventional technique consists initially in grafting the base polyethylene with a silane, by adding radicals using a peroxide. Thereafter, the compound as grafted in that way is subjected to cross-linking by hydrolysis and then to condensation, which requires the presence of water and a condensation catalyst. It should be observed that the catalyst is commonly constituted either by dibutyl tin laurate (DBTL) or by dibutyl tin dilaurate (DBTDL).
With a polyethylene, the silane cross-linking technique nevertheless presents the drawback of being unsuitable for being implemented directly in ambient air if said polyethylene is filled. Unfortunately, in cable making, it is extremely common practice for insulating materials to include fillers. This applies in particular to flame-retardant fillers for improving the behavior of power cables and/or telecommunications cables in the event of fire.
In order to remedy that difficulty, the only solutions presently in use consist in implementing the second cross-linking step of the silane technique either in a pool for 24 hours (h) at 63° C., or else in a sauna for 15 h at 90° C.
Nevertheless, both of those solutions are particularly expensive because of the cost of the extra equipment that is needed, because of the cost of the energy required for operation, and because of the cost of maintaining the installation.
Furthermore, since marking inks do not withstand passing through a pool or a sauna, it is not possible to mark each cable directly on leaving an extruder, and the marking operation must necessarily be performed as an extra operation on leaving the bath of liquid water or of steam. Thus, a consequence of using a pool or a sauna is to complicate quite considerably the industrial fabrication method in terms of logistics, and that again constitutes more extra costs.